Just thought I'd update you all....
Fitted the new alternator..... and exactly the same issues - no tacho, no charge, red light on..... ahh!
scratched head a bit and then took the battery isolator out of circuit, so the positive on the alternator goes to the battery + direct.... worked fine.... 14.4v on output of alternator, tacho working again and charge light out! so thinking alternator was fine... so put the old alternator back (with the changed wiring direct) - issue returned....... now very confused...
put new alternator back and everything working fine.... so have i been really unlucky and had a duff alternator connected to a battery isolator which may also be faulty/have some compatibility issue all of a sudden??? things have been working fine for 3 years until the other day......all very odd as nothing changed...
oh well, its all working fine now, thanks for all the help!
................................................just had a thought, its not true nothing has changed.. the boat did have a smartbank with relay (VSR) between engine/leisure batteries.... i'm now wondering if this may have been masking the problem? I have no idea when this was energised........ just a thought!